In recent years, tactile sensing has rapidly been expanding its presence in the fields of medical treatment, welfare, robot, virtual reality, and so forth.
In the automotive field, for example, it has been typical to embed a pressure sensing element into a seat. This is aimed at prompting a passenger who rides in a vehicle and sits on the seat to fasten a seat belt. More specifically, upon sitting of the passenger on the vehicle seat, a predetermined level or larger load (weight) is applied to the pressure sensing element. Accordingly, a pressure sensor equipped with the pressure sensing element senses the presence of the passenger, and prompts him or her to fasten the seat belt.
Other expected applications of the pressure sensor include those in medical or nursing field.
More specifically, for example, the pressure sensing element embedded in a mattress of a bed is expected to monitor how the weight of a patient or aged person (also referred to as patient, etc., hereinafter) is applied when he or she lies thereon. By the monitoring, it becomes possible to detect that the patient, etc. has been lying on bed in a fixed posture for a long time. Again by the monitoring, a third party can know when to suitably change the posture of the patient, etc. lying on bed, for bedsore prevention.
It is also possible to use the pressure sensor for a walking aid of the patient, etc. More specifically, if an aged person, while waking with a waking aid, embedded with a pressure sensing element, should lose his or her balance, the pressure sensor can detect the unbalanced weight of the aged person as a change in pressure distribution. It is therefore expected to provide falling prevention for the aged person, or detection of falling.
A conventional pressure sensing element, having been widely known, is embodied to have an electro-conductive pressure sensitive resistor formed on a flexible film such as resin film, and a sensor electrode provided so as to oppose with the pressure sensitive resistor. The pressure sensing element of this embodiment is preferred since it is less likely to make the user feel uncomfortable when touched. Specific examples are exemplified by those described in Patent Literatures 1 to 4. Each of the pressure sensing elements disclosed in Patent Literatures 1 to 4 has a pressure sensitive resistor formed on a resin film by printing an electro-conductive material. The pressure sensitive resistor formed by printing is referred to as “pressure sensitive resistor I”, hereinafter.
Patent Literature 1 discloses a pressure sensitive sensor having a film-like pressure sensitive resistor formed by coating an ink composition by printing, and then drying, on a film of polyethylene terephthalate or the like, wherein the ink composition is obtained by dissolving and dispersing an electro-conductive particle, an elastomer particle and a binder into a solvent.
Patent Literature 2 discloses a pressure sensor having a pressure sensitive resistor formed by coating a pressure sensitive resistor paste on a film of polyethylene terephthalate, polyether imide or the like, wherein the paste contains a base polymer, an electro-conductive material such as carbon black, and a filler.
Patent Literature 3 discloses a surface pressure distribution sensor having an opposing electrode film which is composed of a polyethylene terephthalate or polyethylene naphthalate film, and an evaporated pressure sensitive resistor (electro-conductive film) made of gold (Au) or other metal formed on the back surface thereof.
Patent Literature 4 discloses a pressure sensor sheet having a pressure sensitive resistor which is formed by coating a material, obtained by dispersing carbon into a thermosetting resin, onto a base film by screen printing.
Patent Literature 5 discloses a pressure sensor having a pressure sensing part which is configured by a cover film composed of a polyimide film, and a pressure sensitive resistor (pressure sensing film) containing copper oxide or the like formed thereon. This literature describes methods of forming the pressure sensitive resistor, including a method of depositing copper oxide or the like by sputtering or evaporation onto the polyimide film, and a method of oxidizing a copper foil laminated on the polyimide film to thereby convert the surface thereof into copper oxide. The pressure sensitive resistor formed by sputtering, evaporation or the like will also be referred to as “pressure sensitive resistor II”, hereinafter.